HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system. It runs on their PA-RISC range of processors and Intel's Itanium processor, and was also available for latter Apollo/Domain systems. Earlier versions also ran on the HP 9000 Series 200, 300, and 400 computer systems based on the Motorola 68000 series of processors, as well as the HP 9000 Series 500 computers based on HP's proprietary FOCUS processor architecture.

HP-UX differs from other Unix and Unix-like operating systems in that was the first to use access control lists for file access permissions rather than the standard Unix permissions system.